Exhaust system and method for reducing particulate and no2 emissions

ABSTRACT

An exhaust system for reducing particulate and NO 2  emissions from diesel engine exhaust gases. The exhaust system includes a diesel particulate filter, an inlet for receiving the exhaust gases, a first conduit for providing a first fluid connection between the inlet and the diesel particulate filter, and a second conduit for providing a second fluid connection between the inlet and the diesel particulate filter. The exhaust system also includes a diesel oxidation catalyst in the second conduit for catalysing hydrocarbon combustion and the formation of NO 2  and having high I-IC activity and high NO 2  activity, and a fuel injector for injecting fuel upstream of the diesel oxidation catalyst. A valve mechanism for selectively directing exhaust gases from the inlet to the diesel particulate filter through the first conduit or the second conduit is also included. A method for reducing particulate and NO 2  emissions using the exhaust system is also provided.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a National Stage application of InternationalApplication No. PCT/EP2013/050236, filed on Jan. 8, 2013, which claimspriority of Great Britain patent application number 1200230.9, filed onJan. 9, 2012, both of which are incorporated herein by reference intheir entireties.

BACKGROUND

a. Field of the Invention

The present invention relates to an exhaust system for reducingparticulate matter and NO₂ levels in exhaust gases from diesel engines,and to a method for using the system.

b. Related Art

Historically, NOx and particulate emissions have been of particularconcern when developing diesel engines and aftertreatments. Morerecently, legislation and local air quality improvement schemes are nowalso targeting NO₂ reduction.

SCRT is a technology that can control NOx, particulate, CO and HC(hydrocarbon) emissions. However it is difficult also to control NO₂emissions because significant quantities of NO₂ are generated within thesystem in order to facilitate the passive regeneration of the CRT.

SCR has the potential to control NOx and NO₂, but particulate reductionmight be difficult to achieve downstream of this because there islimited energy available in the exhaust gas to enable particulate burn(regeneration) over the filter.

It is known to provide a system in which the DPF regenerates at lowtemperatures. The system is connected downstream of a turbocharger orengine and the exhaust gas is split into a main pipe and a bypass. Thebypass has a heating element and means for injecting diesel fuel intothe exhaust gas stream. A first DOC provides heating through theexothermic heat from catalytic fuel burning. Downstream of the firstDOC, the two pipes join together. Gases from the combined pipes are fedto a second DOC and the DPF. The second DOC also serves to burnhydrocarbons, to increase the temperature to a level at which the filtercan regenerate. Sensors are used to measure temperatures at variouspoints, and relative gas flows through the pipes are adjusted, togetherwith pre-heating using the electric heater, to ensure that gasesreaching the second DOC are above a threshold temperature required forefficient hydrocarbon conversion. The system meters flow through thefirst DOC between 0% to 100% in order to achieve the ideal flow rate formaximum performance. The use of supplementary heating and the flow rateadjustment system has significant cost and development disadvantages.Moreover, because exhaust gases flow through the DOC, which generatesNO₂ it is necessary to use a special oxidation catalyst which isselected for low NO₂ formation but which is less effective than othersfor catalysing HC conversion.

US 2010/0037607 describes a system with parallel exhaust gas flows. Thesystem has a NO oxidation catalyst in one stream and a HC oxidationcatalyst in the other. The HC catalyst acts as a heater when fuel isinjected upstream. This allows selective heating of the DPF withoutheating of the NO catalyst. This is said to optimise the catalyticactivity of the NO oxidation catalyst.

US 2003/0089104 describes an exhaust system which has an exhaust linewith an oxidation catalyser and a catalytically-coated DPF. During a DPFregeneration phase, post-injection of fuel takes place to provideunburned HC to the oxidation catalyst which oxidises the HC. A bypasscircuit and valve arrangement permits exhaust gas with unburned fuel tobypass the oxidation catalyst. When regeneration of the filter istriggered, a proportion of the exhaust gases with injected HC isdiverted straight to the DPF without passing through the oxidiser. Thisarrangement is said to increase the combustion rate of particles trappedin the DPF.

Other prior art systems are described in the following documents:JP2010043577, US2006/117742, US2010/199634, JP2010209783, EP2014348,US2008/155968, EP2309103, JP2006233947, US2011/192143, US2009/277159,US2011/146233, US2010/242438, US2009/178393, WO07/136148, US2009/260346,US2005/031514.

An industry challenge is to develop a DPF system that can regenerate atlow temperature, with low tailpipe NO₂ and, if necessary, in combinationwith other aftertreatment systems, such as those for reducing NOx.

SUMMARY OF THE INVENTION

The invention provides a system and method for reducing tailpipe NO₂emissions while providing for DPF regeneration. According to an aspectof the invention, an exhaust system for reducing particulate and NO₂emissions from diesel engine exhaust gases is provided. The exhaustsystem includes a diesel particulate filter, an inlet for receivingexhaust gases from a diesel engine, a first conduit capable of providinga first fluid connection between the inlet and the diesel particulatefilter, and a second conduit capable of providing a second fluidconnection between the inlet and the diesel particulate filter. Theexhaust system also includes a diesel oxidation catalyst in the secondconduit for catalysing hydrocarbon combustion and the formation of NO₂and having high HC activity and high NO₂ activity. A valve mechanism forselectively directing exhaust gases from the inlet to the dieselparticulate filter through the first conduit or the second conduit isalso included, wherein exhaust gases that pass from the inlet to thediesel particulate filter through the first conduit but not the secondconduit will not encounter a diesel oxidation catalyst. The exhaustsystem also includes a fuel injector for injecting fuel upstream of thediesel oxidation catalyst.

A method for reducing particulate and NO₂ emissions from diesel engineexhaust gases using the exhaust system according to an aspect of theinvention is also provided. According to an aspect of the invention, themethod includes, in normal mode, operating the valve mechanism to directmost or all of the exhaust gases entering the inlet, to the dieselparticulate filter via the first conduit; and in regeneration mode,operating the valve mechanism to direct most or all of the exhaust gasesentering the inlet, to the diesel particulate filter via the secondconduit.

Definitions

When used herein, the following definitions define the stated term:

“CO” is carbon monoxide.

A “CRT” is a Continuously Regenerating Trap system for the removal of PMfrom the exhaust gas stream using a wall-flow filter. The systemoperates passively and is self cleaning. It achieves this by using acatalyst upstream of the filter to produce exhaust gas conditions thatenable the carbon fraction of the PM to be burnt off at typical dieselexhaust gas temperatures.

A “DOC” is a Diesel Oxidation Catalyst, which is used to promote burningof diesel hydrocarbons in an exhaust gas flow.

A “DPF” is a Diesel Particulate Filter, which is used to remove PM fromexhaust gases.

“HC” means hydrocarbon.

“High HC activity” means a catalyst which when fresh will cause orpromote combustion of at least 70% (preferably at least 80%) of HC in anexhaust gas at 300° C.

“High NO₂ activity” means a catalyst which when fresh will cause orpromote conversion of at least 60% (preferably at least 70%) of NO toNO₂ in an exhaust gas at 300° C.

“NO” is nitric oxide.

“NO₂” is nitrogen dioxide, a major contributor to photochemical smog andacid rain. It can be removed from an exhaust gas stream by SCR.

“NOx” is a generic term for all oxides of nitrogen.

“PM” is Particulate Matter, the solid content of exhaust gases,primarily soot (carbon) and ash.

“SCR” is Selective Catalytic Reduction, a process for removing NOx byreducing with a reductant such as ammonia over a catalyst.

“SCRT”® is a combination of SCR and CRT in a single exhaust emissionsreduction system. It is capable of removing NOx, PM, HC and CO. SCRT isa registered trade mark of Johnson Matthey PLC.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be further illustrated, by way of example only,with reference to the following drawings, in which:

FIG. 1 is a schematic representation of an exhaust system in accordancewith an aspect of the present invention;

FIG. 2 is a schematic representation corresponding to FIG. 1, in by-passmode;

FIG. 3 is a graph illustrating decrease in NO₂ output relative to theengine output level in bypass mode;

FIGS. 4 and 5 correspond to FIGS. 2 and 3 for the exhaust system inlight-off mode;

FIGS. 6 and 7 correspond to FIGS. 2 and 3 for the exhaust system inregeneration mode;

FIG. 8 is a graph showing net tailpipe NO₂ reduction over an eight houroperation; and

FIGS. 9 to 13 illustrate alternative embodiments of exhaust systems inaccordance with the present invention.

DETAILED DESCRIPTION

An exhaust system 2 comprises an inlet 6 for receiving exhaust gasesfrom a diesel engine (not shown) and a first conduit 4 connected to aDPF module 10 via an outlet 8. The first conduit 4 is capable ofproviding a first fluid connection between the inlet 6 and the DPF 10. Asecond conduit 12 is capable of providing a second fluid connectionbetween the inlet 6 and the DPF 10. The second conduit 12 houses a DOC14. The DOC catalyst has high HC activity and high NO₂ activity; whenfresh, the DOC converts over 80% HC and over 70% NO₂at 300° C. SuitableDOCs with high HC activity and high NO₂ activity will be well known tothose skilled in the art of exhaust emission control.

A fuel injector 16 is arranged to inject fuel into the exhaust gasstream upstream of the DOC. A valve mechanism 18 is adjustable forselectively directing exhaust gases from the inlet 6 to the DPF 10through the first conduit 4 or the second conduit 12. The fuel injector16 is between the valve mechanism 18 and the DOC 14. In this example,the fuel injector 16 is upstream of the valve mechanism 18 but couldalternatively be in a separate stream within the second conduit 12.Gases which pass through the second conduit 12 encounter the DOC 14before reaching the DPF 10. Gases which pass through the first conduit 4reach the DPF 10 without encountering a diesel oxidation catalyst.Filtered exhaust gases exit the DPF 10 via a tailpipe 28.

In this embodiment, the exhaust system 2 is illustrated in combinationwith an upstream SCR unit 20. The SCR 20 has an injector 22 forintroducing a reductant such as ammonia, and an optional inlet moduleoxidation catalyst 24 and an optional outlet module 26 with slipcatalyst. It will be appreciated that the invention is not limited touse with an SCR. It may be used as a standalone active DPF system or maybe combined with other aftertreatment or exhaust technologies.

Referring to FIG. 2, in bypass (soot filter) operating mode the valvemechanism 18 is open, and exhaust gases entering via the inlet 6substantially bypass the DOC 14 and reach the DPF 10 withoutencountering an oxidation catalyst. This limits or eliminatesNO₂production over the DOC. A reduction in engine-out NO₂ occurs overthe DPF 10 by passive reduction of NO₂ over accumulated particulatematter. The decrease in NO₂ relative to the engine-out level isillustrated in FIG. 3; both soot and NO₂ levels are reduced by the DPF.

Referring now to FIG. 4, in light-off mode, the valve mechanism 18closes off the path through the first conduit 4, diverting exhaust gasesthrough the second conduit 12 and DOC 14. Combustible components of thehot exhaust gases, notably CO and HC, are catalytically oxidised overthe DOC 14. We have found that it is during this brief light-off periodthat the most significant quantity of NO₂ is produced, as shown in FIG.5.

When the DOC achieves light-off, the system switches to regenerationmode, and fuel is injected via the fuel injector 16 to oxidise over theDOC (FIG. 6). The resulting exotherm increases the temperature of theexhaust gas sufficiently to combust the organic fraction of PM(predominantly carbon/adsorbed HCs). This enables burning of the PMaccumulated within the DPF and regeneration of the DPF filteringcapacity. Combustion of the organic fraction of PM on the DPF couldoptionally be catalysed by use of a fuel-borne catalyst so that thetemperature required to initiate PM combustion is lowered, requiringless energy to start regeneration. In this embodiment, the fuel injector16 is arranged and adapted to direct most or substantially all of theinjected fuel directly into the second conduit 12. By ‘directly’ we meanwithout substantial axial travel in the first conduit 4. As illustrated,the fuel injector 16 is positioned directly opposite the entrance to thesecond conduit 12 where it branches from the first conduit 4. Theinjected fuel is directed straight across the first conduit 4 into thesecond conduit 12. The fuel injector 16 could alternatively be locatedin a wall of the second conduit 12 upstream of the DOC 14.

As shown in FIG. 7, NO₂ production is suppressed when fuel injectiontakes place. The catalyst has preferred selectivity towards HC oxidationrather than NO oxidation.

Tailpipe NO₂ output is relatively high during the light-off mode, asillustrated in FIG. 5, but this mode is operated for only a smallproportion of the duty cycle. This offsets some, but not all, of the NO₂reduction that occurs during the rest of the duty cycle in bypass (sootfilter) mode. Results for an eight hour shift at steady state operationare shown in FIG. 8. Cumulative tailpipe NO₂ is substantially reducedwith respect to cumulative engine-out NO₂. The invention provides aregenerative DPF system with low tailpipe NO₂. The system may regenerateat low temperatures, particularly if a fuel-borne catalyst is used, andit may be used in combination with other aftertreatment systems, such asthose for reducing NOx.

In the embodiment illustrated in FIG. 1, the valve mechanism 18 is asimple butterfly valve located in the first conduit. When the valve 18is open, most exhaust gases pass through the first conduit to the DPF.

The relatively low gas flow through the second conduit 12 can beeliminated completely by use of a second valve 19 in the second conduit,as illustrated in the embodiment shown in FIG. 9. The first valve 18 andthe second valve 19 are independently controllable to allow exhaustgases to flow through only the first conduit 4 or the second conduit 12according to the operating mode.

Referring now to FIG. 10, another embodiment is illustrated, which usesa three-way valve 18 at the junction of the conduits 4,12 forselectively directing exhaust gases through either or both conduits.

The first conduit 4 and the second conduit 12 may be arranged in anyconvenient manner. In the embodiment illustrated in FIG. 11, theconduits 4,12 are coaxial. A valve 18 in the central second conduit 12selectively permits exhaust gas flow through the DOC 14 during light-offand regeneration modes. Because the second conduit 12 is aligned withthe exhaust gas inlet 6, gases quickly reach the DOC 14 when the valveis opened. Accordingly, this embodiment provides fast light-off whenlight-off mode is selected. It will be understood that the DOC 14 couldalternatively be located in the outer annular passage, which wouldfunction as the second conduit. It will be further understood that thevalve mechanism 18 could alternatively be provided in the outer annularpassage.

The embodiment illustrated in FIG. 12 is a similar fast light-offarrangement to that of FIG. 11, but with an alternative valve mechanism18 that allows all exhaust gases to be diverted through either the innerconduit or the outer annular conduit. The valve mechanism 18 comprises afirst valve disc 18 a and a second valve disc 18 b, each of which isprovided with at least one outer aperture 30 and at least one inneraperture 32 which correspond respectively with the first conduit 4 andthe second conduit 12. Relative rotation of the valve discs selectivelybrings the outer apertures 30 into alignment and the inner apertures 32out of alignment. Further relative rotation of the valve discsselectively brings the outer apertures 30 out of alignment and the innerapertures 32 into alignment. When the inner apertures 32 are aligned,the central (second) conduit 12 is opened, and when the outer apertures30 are aligned, the annular (first) conduit 4 is opened. At intermediatealignments the valve mechanism 18 optionally prevents gas flow througheither conduit.

As illustrated, the DOC 14 is located in the inner conduit whichfunctions as the second conduit 12, while the annular conduit functionsas the first conduit 4. It is appreciated that the DOC couldalternatively be provided in the annular conduit.

It will be appreciated that various sensors and control systems may beemployed to optimise performance of the exhaust system. For example, thetemperature of exhaust gases may be monitored at or downstream of theDOC to determine when light-off occurs, and to trigger fuel injection ator after light-off. Accurate control of the fuel injection rate ispreferred in order to limit unburned fuel passing through the DOC. Thepressure difference over the DPF may be monitored, and regeneration modetriggered when a threshold Δp value is reached, corresponding to athreshold level of channel blockage within the DPF.

The system may be open loop, closed loop, feedforward, feedback or useother suitable monitoring and control methods.

The system is simple in terms of hardware and control, providing costand operating advantages. For example, whereas the prior art systemrequires control of flow through the first catalyst anywhere between 0%and 100% in order to achieve desired flow rates through the DOC, this isnot necessary in the present system. The valve mechanism can simply beswitched between zero and maximum flow depending on the operating moderequired.

The system uses fewer components than prior art systems, and operatesdifferently to achieve both DPF regeneration and low NO₂ levels at thetailpipe. The described prior art system cannot achieve low tailpipe NO₂because there will always be exhaust gas flowing through a catalyst thatwill generate NO₂.

The prior art system requires a special oxidation catalyst selected forgood HC conversion and low NO₂ formation. An advantage of the presentsystem is that it can use a very active catalyst that typically has bothhigh HC and NO₂ conversion, lighting off at lower temperatures andhaving higher efficiency. The layout and operation of the system is suchthat low tailpipe NO₂ is achieved relative to NO₂ going into the system.

It is appreciated that certain features of the invention, which are, forclarity, described in the context of separate embodiments, may also beprovided in combination in a single embodiment. Conversely, variousfeatures of the invention which are, for brevity, described in thecontext of a single embodiment, may also be provided separately, or inany suitable combination.

What has been described above are preferred aspects of the presentinvention. It is of course not possible to describe every conceivablecombination of components or methodologies for purposes of describingthe present invention, but one of ordinary skill in the art willrecognize that many further combinations and permutations of the presentinvention are possible. Accordingly, the present invention is intendedto embrace all such alterations, combinations, modifications, andvariations that fall within the spirit and scope of the appended claims.

What is claimed is:
 1. An exhaust system for reducing particulate andNO₂ emissions from diesel engine exhaust gases, the system comprising: adiesel particulate filter; an inlet for receiving exhaust gases from adiesel engine; a first conduit capable of providing a first fluidconnection between the inlet and the diesel particulate filter; a secondconduit capable of providing a second fluid connection between the inletand the diesel particulate filter; a diesel oxidation catalyst in thesecond conduit, the diesel oxidation catalyst catalysing hydrocarboncombustion and the formation of NO₂ and having high HC activity and highNO₂activity; a valve mechanism for selectively directing exhaust gasesfrom the inlet to the diesel particulate filter through the firstconduit or the second conduit; wherein exhaust gases that pass from theinlet to the diesel particulate filter through the first conduit but notthe second conduit will not encounter a diesel oxidation catalyst; and afuel injector for injecting fuel upstream of the diesel oxidationcatalyst.
 2. The exhaust system according to claim 1, wherein the fuelinjector is upstream of the valve mechanism.
 3. The exhaust systemaccording to claim 1, wherein the fuel injector is arranged and adaptedto direct most or substantially all of the injected fuel directly intothe second conduit.
 4. The exhaust system according to claim 1, furthercomprising an exhaust gas aftertreatment unit upstream of the inlet. 5.The exhaust system according to claim 4, wherein the aftertreatment unitis an SCR unit.
 6. The exhaust system according to claim 1, wherein thefirst conduit and the second conduit are substantially concentric. 7.The exhaust system according to claim 6, wherein the second conduit isdisposed centrally within the first conduit.
 8. The exhaust systemaccording to claim 6, wherein the first conduit is disposed centrallywithin the second conduit.
 9. A method for reducing particulate and NO₂emissions from diesel engine exhaust gases using the system of claim 1,the method comprising: in normal mode, operating the valve mechanism todirect most or all of the exhaust gases entering the inlet, to thediesel particulate filter via the first conduit; in regeneration mode,operating the valve mechanism to direct most or all of the exhaust gasesentering the inlet, to the diesel particulate filter via the secondconduit.
 10. The method according to claim 9, further comprising, inregeneration mode, injecting fuel via the fuel injector to oxidise onthe diesel oxidation catalyst and increase the temperature of exhaustgases reaching the diesel particulate filter to burn at least someparticulate matter accumulated therein.
 11. The method according toclaim 10, further comprising monitoring the temperature of exhaust gasesat or downstream of the diesel oxidation catalyst to determine whenlight-off occurs, and commencing the fuel injection at or after lightoff.
 12. The method according to claim 9, further comprising providing acatalyst to the diesel engine fuel, to catalyse the combustion ofparticulate matter on the diesel particulate filter.
 13. The methodaccording to claim 9, further comprising monitoring a pressuredifference over the diesel particulate filter and triggeringregeneration mode when a preset pressure difference is reached. 14.(canceled)
 15. (canceled)